1. Field of the Invention
The present invention relates to a display data obtaining apparatus and a display data obtaining method with which a signal of an acoustic wave from an inner portion of a subject to be examined (hereinafter, referred to as “subject”) irradiated with light is detected, and the detected signal is processed to obtain information of the inner portion of the subject.
2. Description of the Related Art
In medical fields, there has been actively developed an optical imaging apparatus for irradiating a living body with light emitted from a light source such as a laser, and for displaying on a display information of an inner portion of the living body which is obtained based on light entering the living body. An example of the optical imaging technology is photoacoustic tomography (PAT). In the photoacoustic tomography, a living body is irradiated with pulsed light from the light source so as to detect an acoustic wave (typically ultrasound) generated from a living tissue having absorbed the energy of the pulsed light being propagated and diffused in the inner portion of the living body. That is, an elastic wave, which is generated when the examined region having absorbed the energy of the irradiated light is momentarily expanded, is received as a detected signal by an acoustic wave detector (also called a probe or transducer) using a difference in an optical energy absorption factor between an examined region (for example, tumor) and the other tissue. When the detected signal is analyzed and processed, an optical characteristic distribution, in particular, an initial sound pressure distribution or absorption coefficient distribution of the inner portion of the living body may be obtained. When the measurement described above is performed with light beams having various wavelengths, the obtained information may also be used to quantitatively measure specific substances in the inner portion of the living subject, for example, a concentration of hemoglobin contained in blood and an oxygen saturation of blood.
There has been known a problem in that, the photoacoustic tomography apparatus causes a spatial sensitivity distribution specific to the apparatus, when an acoustic wave measurement region is insufficient compared to an information acquisition region, for example, when the acoustic wave may be received from not in all directions of a subject but in only a specific direction (“128-channel laser optoacoustic imaging system (LOIS-128) for breast cancer diagnostics”, Sergey A. Ermilov, Andre Conjusteau, Ketan Mehta, Ron Lacewell, P. Mark Henrichs, and Alexander A. Oraevsky, Proceedings of SPIE, Vol. 6086, 608609, 2006). A spatial sensitivity distribution is a distribution that exhibits fluctuations in measurement sensitivities for respective locations (sound source positions) in a measurement object. The fluctuation in sensitivities, which depend on locations, occurs because an information amount which may be used for each voxel is changed depending on a relative voxel position difference between the acoustic wave detector and a sound source even in a case where completely the same target parameter (for example, initial sound pressure) is measured. Therefore, results obtained by the detection are displayed using contrasts, which are changed depending on the locations in the measurement object. As a result, the quantitativity of obtained information is deteriorated. The relative position between the acoustic wave detector and the sound source is changed for every apparatus, and hence a spatial sensitivity distribution contrast difference is also changed, respectively. In order to solve the problem described above, it is known that it is important to detect an acoustic wave in all directions of 360° around the subject to obtain a sufficient information amount (“Photoacoustic Imaging in biomedicine”, M. Xu and L. V. Wang, Review of Scientific Instruments, 77, 041101, 2006).
However, when a large object, for example, breasts of a human are subjected to examination, it is difficult to measure an acoustic wave in various positions around the entire subject, and hence inner information of a living body cannot be obtained by using a sufficient amount of information. In other words, when such objects are subjected to examination, the method of Xu et al. is not necessarily practical to eliminate the deterioration of the quantitativity of the obtained information.